Cultivar and developmental differences in S-protein concentration and self-incompatibillty in the Japanese pear

Cultivars of the Japanese pear [Pyrus pyrifolia (Burm.) Nakai] have variabl e degrees of self-incompatibility (SI) and can be classified into at least three groups: strong, intermediate, or weak SI; as shown by the extent of s elf-pollen tube growth in the style, and the percentage of fruit set follow ing self-pollination. Following self-pollination, the elongation of pollen tubes in the detached styles of 'Kosui' and 'Kikusui' became increasingly s uppressed from 4 days before anthesis (-4 DAA) to 2 days after anthesis (2 DAA). Tube growth of 'Kosui' was more suppressed than that of 'Kikusui' dur ing this period. In 'Osa-Nijisseiki', however, the rate of tube growth did not vary with stage of stylar development, from -8 to 2 DAA. Pollen tubes e longated much better after cross-pollination than after self-pollination at all stages tested, and the extent of the elongation increased as the style s matured. The concentration of total S-protein (sum of two S-proteins per buffer-soluble protein) increased with stylar development, but the rate of increase varied with the cultivar, The rate was significantly greater in th e strongly self-incompatible 'Kosui' than in the moderately self-incompatib le 'Kikusui', and was slowest in the weakly self-incompatible 'Osa-Nijissei ki' at all developmental stages. During stylar maturation, the concentratio n of S-4-protein, which is common in all cultivars, was highest in 'Kosui', followed by 'Kikusui' and 'Osa-Nijisseiki'. Thus, the cultivar differences in SI expression in the Japanese pear are determined about -4 DAA and appe ar to be regulated, in part, by the concentration of S-proteins produced in the style.